Single-Nucleus Sequencing of Silkworm Larval Brain Reveals the Key Role of Lysozyme in the Antiviral Immune Response in Brain Hemocytes

Introduction: The brain is considered as an immune-privileged organ, yet innate immune reactions can occur in the central nervous system of vertebrates and invertebrates. Silkworm (Bombyx mori) is an economically important insect and a lepidopteran model species. The diversity of cell types in the silkworm brain, and how these cell subsets produce an immune response to virus infection, remains largely unknown. Methods: Single-nucleus RNA sequencing (snRNA-seq), bioinformatics analysis, RNAi, and other methods were mainly used to analyze the cell types and gene functions of the silkworm brain. Results: We used snRNA-seq to identify 19 distinct clusters representing Kenyon cell, glial cell, olfactory projection neuron, optic lobes neuron, hemocyte-like cell, and muscle cell types in the B. mori nucleopolyhedrovirus (BmNPV)-infected and BmNPV-uninfected silkworm larvae brain at the late stage of infection. Further, we found that the cell subset that exerts an antiviral function in the silkworm larvae brain corresponds to hemocytes. Specifically, antimicrobial peptides were significantly induced by BmNPV infection in the hemocytes, especially lysozyme, exerting antiviral effects. Conclusion: Our single-cell dataset reveals the diversity of silkworm larvae brain cells, and the transcriptome analysis provides insights into the immune response following virus infection at the single-cell level.